I'm having an intermittent problem (every 3 times I drive for a long time) that a slight clunking noise is heard and felt though the brake pedal every 3 seconds or so when my car is warm. It sounds similar to clicking your tongue against the bottom of your mouth. I opened the hood and could feel the clunking noise when putting my hand on the master cylinder.
The braking performance hasn't been impaired at all. It's just a bit embarrassing. I checked the brake fluid and the level looks fine.
Any ideas what could this be? Is it a vacuum leak? Air in the lines? Master cylinder on its way out?